With recent trends towards higher density of integration of electronic instruments, a need has arisen for flexible circuit board having passive elements (inductor and coil antenna, for example) densely mounted thereon. Higher density of integration on the flexible circuit board may be embodied by stacking a plurality of resin bases, each having an interconnect pattern formed on one surface thereof, while placing an adhesive layer in between to thereby form a multilayer structure, typically as described in Patent Document 1 (Japanese Laid-Open Patent Publication No. H11-54934).
Prior art documents regarding the multilayer structure, besides Patent Document 1, may be exemplified by Patent Document 2 (Japanese Laid-Open Patent Publication No. 2008-103640) and Patent Document 3 (Japanese Laid-Open Patent Publication No. 2007-096121). The multilayer structure disclosed in Patent Document 2 has a structure in which circuit boards each having an interconnect pattern (electro-conductive pattern) and multilayer resin layers are alternately stacked, and each multilayer resin layer contains a first layer composed of a thermoplastic resin, a second layer composed of a thermosetting resin, and a connective conductor extended through the first layer and the second layer.
On the other hand, Patent Document 3 discloses a circuit board configured by an insulating base, having a conductor pattern formed on the surface thereof and composed of a thermoplastic resin, and an electro-conductive paste filled in via holes formed to extend through the insulating base. The multilayer structure disclosed in Patent Document 3 is configured by collectively or sequentially stacking a plurality of such circuit boards by heat bonding.    [Patent Document 1] Japanese Laid-Open Patent Publication No. H11-54934    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2008-103640    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2007-096121